TERROIR STUDY OF HONEYCRISP APPLES IN WISCONSIN, USA: INSIGHTS FROM SOIL, LEAF, AND FRUIT CHEMISTRY

Abstract

Wisconsin agriculture forms an integral part of the state's economy. Although cranberries and cherries serve as the state’s most exported fruit, a lesser known but popular local crop is apples. Terroir, the study of environmental factors on plant growth and the fruits borne by those plants, is often applied to wine grape studies, but has recently branched out to other agricultural fields. Terroir relates acutely to Wisconsin’s apple growing agriculture in that the state’s varying bedrock geology interacts with the growth and products of the apple trees. This research focuses on the soil and bedrock aspect of Honeycrisp apple terroir. By analyzing soil, leaf, and apple samples, we seek to link chemical elemental signatures in the soil to the tissue in the apple trees and, ultimately, the fruit. We visited and sampled six apple orchards around Wisconsin, exploring four different bedrock types. The sampling focused on trees over 10 years old that produce Honeycrisp apples grown on M7 rootstock. Two soil pits were dug at each site from which soil samples of the A-Horizon and the B-Horizon were collected. Additionally, leaf samples were taken from trees immediately adjacent to the soil pits. Grain size of the soils was measured using a Malvern Mastersizer. Grain size helps classify soil on a sand-silt-clay ternary soil plot. The clay content of the soil is especially influential on soil water holding capacity. Mineralogy of the soils was determined by x-ray diffraction (XRD) and bulk soil chemistry was measured by x-ray fluorescence (XRF). A soil nutrient package was measured at A&L Canada Laboratories Inc. In addition, leaves and apples were sent to A&L Canada Laboratories Inc. for standard elemental analysis via inductively coupled plasma-mass spectrometry (ICP-MS). Chemical data elucidates the transfer of Na and Ca from the soil into the tree roots and ultimately the leaves and the apple. Principal Component Analysis (PCA) diagrams show that either Mg or Ca in soil and apples are the main discriminators of soil groups, though the PCA of leaf chemistry showed less significant soil group discrimination.